138 DR. F. OSWALD OX THE [June I914,. 



area which they may once have covered. Prohably this extensive 

 denudation was not so much subaerial, as due to the lake having 

 stood for a long period at a level of nearly 4100 feet above the sea : 

 that is, about 328 feet above the present level, as indicated by 

 the evidence which I adduce later (p. 146). 



IV. Basemext-Floor of the Miocexe Seeies. 



The amphibolite at Nira is traversed by a vein of quartz a foot 

 thick (running in a "vt est-north-west to east-south-east direction), 

 which passes up into the overlying crimson clay, and extends 

 also laterally. It is evidently owing to associated ferruginous 

 solutions that both the amphibolite and the originally yellow clay 

 are traversed by a network of veins with dark crimson staining. 

 In the gully of South Nira the amphibolite passes upwards into 

 about 20 feet of rotten rock (a grey, sandy, brown-veined clay), 

 covered by a foot of brown ferruginous sandstone, probably repre- 

 senting the ancient ironstone soil or murrain which cloaked the 

 older rocks prior to the transgression of the lake in Miocene times. 

 This basement-floor of amphibolite is not level, but rises in small 

 irregular bosses and ridges, with a north-westerly strike, covered 

 by the crimson basement-clay (No. 37) ; on the south-eastern and 

 eastern side of the South Nira Gully it rises in a rounded knoll, 

 which is directly overlain and flooded hj the nepheline-basalt from 

 Nira Hill. 



This basement is not actually visible at Kachuku, but the cliff 

 of quartz-ironstone breccia at West Kachuku is probably the old 

 soil of the amphibolite, somewhat remanie by the transgressive 

 waters of the lake, and it is overlapped by the crimson clay 

 (No. 37). It forms a low bare ridge with north-westerly strike, 

 stretching towards Nira, and it is free from the envelope of black 

 cotton-soil extending on each side. Farther east the crimson clay, 

 cropping out about 150 yards from the base of the cliff of East 

 Kachuku, overlies a similar quartz-ironstone breccia, full of par- 

 ticles of angular quartz (up to 2 inches in longest diameter) and 

 of partly rounded ironstone. Lower down the slope a pale greenish- 

 grey phyllite l crops out from under this ironstone breccia, having a 

 foliation directed 30° south- south-west and a north-north-west to 

 south-south-east strike. It forms the western and southern selvage 

 to the rounded hill of Rabur, and borders the alluvial plain of the 

 Kuja. Rabur rises rather abruptly out of the gentle slope of 

 the schistose area, and consists of a dark-green fine-grained zoisite- 

 hornblende rock. 2 It is probably an altered dolerite, and weathers 

 into rounded blocks, contrasting with the splintery chloritic calc- 



1 It consists essentially of calcite and chlorite, with abundant leuco- 

 xene in drawn-out lenticles and a little quartz. 



2 The rock is chiefly composed of green hornblende in sheaves of bladed 

 crystals, together with zoisite and some quartz in very fine aggregates. 

 Ilmenite is rather abundant , often altered marginally to leucoxene; 

 p y r i t e and e p i d o t e are accessory. 



